SUPERSTRINGS AND OTHER THINGS On the heels of this paper, Einstein submitted for publication another important paper on molecular motion, where he explained the erratic, zigzag motion of individual particles of smoke. Again, always seeking the fundamental, Einstein was able to show that this chaotic motion gives direct evidence of the existence of molecules and atoms. My main aim, he wrote later, was to find facts that would guarantee as far as possible the existence of atoms of definite finite size Almost a century earlier, Joseph von Fraunhofer, an illustri- ous German physicist, discovered that the apparent continuity of the sun's spectrum is actually an illusion s seemingly unre- lated discovery was actually the beginning of the long and tortu- ous road toward the understanding of the atom. The eleventh and youngest child of a glazier, Fraunhofer became apprenticed to a glass maker at the age of twelve. Three years later, a freak acci- dent turned the young lads life around; the rickety boarding ouse he w as living in collapsed and he was the only survivor Maximilian i the elector of bavaria rushed to the scene and took pity of the poor boy. He gave the young man eighteen ducats. With this small capital, Fraunhofer was able to buy books on optics and a few machines with which he started his own glass-working shop. While testing high-quality prisms Fraunhofer found that the spectrum formed by sunlight after it passed through one of his prisms was missing some colors; it was crossed by numerous minuscule black lines, as in figure 1.3 (color plate). Fraunhofer, intrigued, continued studying the phenomenon, measuring the position of several hundred lines He placed a prism behind the eyepiece of a telescope and discov ered that the dark lines in the spectrum formed by the light from the stars did not have quite the same pattern as that of sunlight He later discovered that looking at the light from a hot gas ough a prism produced a set of bright lines similar to the pattern of dark lines in the solar spectrum. Today we know that the gaps in the spectrum that Fraun- hofer discovered are a manifestation of the interaction between light and matter. The missing colors in the spectrum are deter mined by the atoms that make up the body emitting the light In the spring of 1925 a twenty-four-year old physicist named Werner Heisenberg, suffering from severe hay fever, decided to take a two week vacation on a small island in the north sea 6
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Physics: The Fundamental Science away from the flowers and the pollen. During the previous year, Heisenberg had been trying to understand this interaction between light and matter, looking for a mathematical expression for the lines in the spectrum. He had decided that the problem of the relationship between these lines and the atoms could be ana- lyzed in a simple manner by considering the atom as if it were an oscillating pendulum. In the peace and tranquility of the island, Heisenberg was able to work out his solution, inventing the mechanics of the atom. Heisenberg's new theory turned out to be extremely powerful, reaching beyond the original purpose of btaining a mathematical expression for the spectral lines In 1984, this idea of thinking about the atom as oscillations took a new turn. John Schwarz of the California Institute of Technology and Michael B Green of the University of London proposed that the fundamental particles that make up the atom re actually oscillating strings. The different particles that scien- tists detect are actually different types or modes of oscillation of these strings, much like the different ways in which a guitar string vibrates. This clever idea, which was incredibly difficult to implement, produced a theory of enormous beauty and power which explains and solves many of the difficulties that previous theories had encountered. The current version of the theory, called superstring theory-which we will study in more detail in chapter 25-promises to unify all of physics and help us understand the first moments in the life of the universe. Still far from complete, superstring theory is one of the most active areas of research in physics at the present time In all these cases, the scientists considered a phenomenon of nature, simplified its description, constructed a theory of its beha- vior based on the knowledge acquired by other scientists in the ast, and used the new theory not only to explain the phenom enon, but also to predict new phenomena. This is the way physics is done. This book shows how we can also do physics, and share in its excitement The scientific method: learning from our mistakes In contrast to that of many other professionals, the work of a cientist is not to produce a finished product. No scientific
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SUPERSTRINGS AND OTHER THINGS theory will ever be a correct, finished result. There could be no fairer destiny for any .. theory, wrote Albert Einstein,"than that it should point the way to a more comprehensive theory in which it lives on, as a limiting case Science is distinguished from other human endeavor by its empirical method, which proceeds from observation or experiment The distinguished philosopher of science Karl R Popper said that the real basis of science is the possibility of empirical disproof A scientific theory cannot be proved correct. It can, however, be disproved. According to the scientific method a scientist formulates a eory inspired by the existing knowledge. The scientist uses this new theory to make predictions of the results of future experiments. If when these experiments are car rried out the pre- dictions disagree with the results of the experiments the theory is disproved; we know it is incorrect. If, however, the results agree with the forecasts of the theory, it is the task of the scientists to draw additional predictions from the theory, which can be tested by future experiments. No test can prove a theory, but any single test can disprove it In the 1950s, a great variety of unpredicted subatomic par ticles discovered in laboratories around the world left physicists bewildered. The picture that scientists had of the structure of matter up to the 1940s-as we will learn in more detail in chapters 7 and 8-was relatively simple and fairly easy to understand matter was made of atoms, which were composed of a tir nucleus surrounded by a cloud of electrons. The nucleus, in turn, was made up of two kinds of particles, protons and neu- trons. The new particles being discovered did not fit this simple scheme. Two theories were formulated to explain their existence The first one proposed a particle democracy, in which no par- ticle was any more fundamental than any other. This theory was so well received by the scientific community in the United Sates that one of the proponents of the second theory, Murray Gell- Mann of the California Institute of Technology decided to publish his paper in a European journal where he felt the opposition to his new ideas would not be so great. Gell-Mann and independently George Zweig, also of Caltech, proposed that many of the grow ing number of particles and in particular the proton and the neutron were actually made up of smaller, indivisible particles 8
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Physics: The Fundamental science which Gell-Mann called quarks. Different combinations of quarks, in groups of two or three, were responsible for many of these par- ticles. According to their theory, the growing number of new par ticles being discovered was not a problem anymore. What mattered was that the objects of which these particles were made of were simple and small in number Which theory was correct? In 1959 Stanford University built Q large particle accelerator which, among other things,could etermine whether or not quarks existed. Seven years later, experiments carried out at the Stanford Linear Accelerator Laboratory, SLAC, allowed physicists to determine the presence of the quarks inside protons and neutrons. Since then, many experiments have corroborated the Stanford results; the quark is accepted today as one of the fundamental constituents of matter and the"particle democracy"theory is no longer viable We shall see in the final chapters of this book that these new theories of matter are far from complete. Nevertheless, the knowl- edge obtained from these theories has given us not only a better understanding of the universe we live in but has also produced the modern technological world based largely on the computer We can summarize the scientific method by saying that can learn from our mistakes. Scientific knowledge progresses by guesses, by conjectures which are controlled by criticism, by itical tests. These conjectures or guesses may survive the tests; but they can never be established as true. The very refutation of a theory, writes Popper, is always a step forward that takes us nearer to the truth. And this is how we learn from our Physics and other sciences Physicists often become interested in Phenomena normally studied by scientists in other scientific disciplines, and apply their knowledge of physics to these problems with great success The recent formulation of the impact theory of mass extinctions a good illustration of physicists becoming involved in other scien- tific fields and of the way working scientists apply the scientific method to their work
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SUPERSTRINGS AND OTHER THING In 1980, the Nobel prize winning physicist Luis Alvarez and his son Walter, a professor of geology at the University of Califor- nia at Berkeley, reported in a paper published in the journal Science that some 65 million years ago a giant meteorite crashed into the earth and caused the extinction of most species. The inosaurs were the most famous casualties. Alvarez and his collaborators based their theory on their study of the geological record. Walter Alvarez had told his father that the 1-cm-thick clay layer that separates the Italian limestone deposits of the Cretaceous period -the last period of age of reptiles-from those of the Tertiary period -the first period of the age of mammals, HOvRE BEN RECALLED. Hc5 GOING TO TRY MAALS Figure 1.4. An unorthodox theory of the extinction of the di Cartoon by Sydney Harris
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